Frequency conversion devices, such as mixers and quadrature modulators, convert the frequency of an input signal by mixing it with a Local Oscillator (LO) signal. In some cases, the LO signal may leak to the output of the device and appear as a spurious signal. This effect is referred to as LO leakage, and is generally undesirable.
Various techniques for reducing LO leakage are known in the art. For example, U.S. Pat. No. 5,012,208, whose disclosure is incorporated herein by reference, describes a quadrature modulator comprising a circuit for compensating for an LO leak signal in an output signal of the modulator. The circuit adds different compensation voltages to two modulation signals. To provide automatic compensation of the LO leak signal, the quadrature modulator comprises a regulating circuit that regulates the intensity of the compensation voltages irrespective of each other, on the basis of the correlation between the amplitude variation in the output signal and the corresponding modulation signal.
U.S. Pat. No. 6,711,396, whose disclosure is incorporated herein by reference, describes a method for reducing LO leakage in radio frequency mixers. Compensating circuits are used to reduce or eliminate unwanted leakage from the mixer output signal by controlling the current in individual branches of a balanced input circuit of the mixer. The compensating circuits are made of different combinations of resistor, transistor and current sink components. By using the compensating circuits with a radio frequency mixer, DC current differential occurring between two output nodes of the input circuit are reduced or eliminated.
U.S. Pat. No. 4,355,420, whose disclosure is incorporated herein by reference, describes a frequency converter having first, second and third terminals that receive a first signal having a first frequency band, a LO signal, and a second signal having a second frequency band. The first signal and the LO signal are mixed to generate the second signal. A first branching circuit is connected between the second terminal and the frequency mixer, for branching part of the LO signal. A first part of the branched LO signal, which is supplied from the first branching circuit, and a first leaked LO signal are applied from the frequency mixer to the first terminal. The phase and amplitude of the first branched LO signal are adjusted so that the first branched LO signal and the first leaked LO signal have equal amplitude and are reversed in phase to cancel each other.
U.S. Pat. No. 6,167,247, whose disclosure is incorporated herein by reference, describes a method for reducing the level of an undesired signal, generated through a frequency converter device. A feedback correction loop, for a radio frequency transmitter or receiver, reduces an undesired LO leak generated through a single-input frequency converter device, such as a mixer. A nulling circuit is also implemented for a multiple-input frequency converter device, such as a quadrature modulator.
U.S. Pat. No. 7,280,805, whose disclosure is incorporated herein by reference, describes a method for calibrating a Radio Frequency (RF) transmitter. The method includes inputting a test tone to the RF transmitter. LO leakage calibration is performed on an output of the RF transmitter with the test tone inputted thereto, in order to determine a minimum LO leakage. Thereafter, sideband image calibration is performed on the output of the RF transmitter with the test tone inputted thereto, in order to determine a minimum sideband image.
Lanschützer et al. describe a technique for adaptive compensation of LO leakage and direct Current (DC) offset, in “A Digital Adaptive Feedback Loop for Local Oscillator Leakage Cancellation and DC Offset Compensation,” Proceedings of the 2002 Asia-Pacific Microwave Conference (APMC 2002), Kyoto, Japan, Nov. 19-22, 2002, which is incorporated herein by reference. The technique is intended for use in direct up-conversion transmitters.